62 DECEMBER 2014 • WORLD AQUACULTURE • WWW.WAS.ORG Single Cell Proteins: a Novel Approach in Aquaculture Systems Vivekanand Bharti, P. K. Pandey1 and Satish Kumar Koushlesh T he rapid growth in aquaculture and scarcity of fishmeal contribute to the high prices of aquafeeds. Currently fishmeal and soybean meal are the most common feed ingredients in aquafeeds, representing 40-60 percent of total variable costs. A serious challenge remains in reducing input costs through reducing the level of fishmeal in feeds and palliating pressure on natural fish populations. Aquaculture producers are seeking alternatives to reducing input costs to enhance profitability. Furthermore, consumers have become more health conscious. The practice of organic farming in aquaculture is one current response to these drivers. Among various alternative strategies available today, the exploitation of microorganisms in aquaculture minimizes the use of antibiotics and represents a source of high-quality protein at low cost. Microorganisms in aquaculture production are used as live organisms, killed organisms or extracted nutrients in aquafeed. Microorganisms such as algae, bacteria, yeast, molds and higher fungi — grown in large-scale culture systems or in biofilm/biofloc systems — can be used in aquaculture. The application of microorganisms or its products as single-cell protein (SCP) in aquafeeds is one of the best approaches to enhance aquaculture sustainability. Single Cell Protein Single cell protein broadly refers to microbial biomass or protein extract used as a food or feed additive. Yeast has been used as a source of protein in human food from ancient times. Microorganisms contain high levels of protein, fats, carbohydrates, nucleic acids, vitamins and minerals (Table 1). The importance of yeast and other microorganisms has been realized and further research has been directed toward its utilization in aquafeeds. Efficient exploitation of SCP of microbial origin can replace up to 50 percent of the fishmeal (Dhevendaran et al . 2013). Substrates such as whey starch, cellulose hydrocarbon, alcohols and molasses have been used to produce SCP. There are several technologies for production of SCP at the industrial level and the basic steps are illustrated in Figure 1. Technically the production of microbial biomass is achieved either by a submerged or solid state fermentation process. Production of Single-Cell Protein Microorganisms can use inexpensive inorganic compounds, such as ammonium salts and carbon, to generate energy for metabolism and growth. Waste inorganic resources can be converted into protein biomass through rapid growth of microorganism on the substrate. Cheap and abundantly available agricultural and industrial wastes can be used for SCP production. Solid state fermentation utilizes solid substrates such as bran, bagasse and paper pulp while submerged fermentation uses free-flowing liquid substrates, including molasses and broths. Sterilization is not required because pathogenic microorganisms are not used for production of SCP for aquafeeds. Sources of SCP in Aquaculture Numerous microorganisms with a high nutritional value can be used in aquafeeds and these are discussed below. Algae. Algae such as Chlorella sp., Chondrus sp., Scenedesmus sp., Spirulina sp. and Porphyrium sp. can be applied as feed supplements or substitutes for conventional protein sources (fishmeal and soybean meal) in aquaculture. The application of algal biomass in animal feeds, including aquafeeds, is about 30 percent of the current world algal production. Euglena gracilis is one of the most preferred algae for SCP production because it has a high protein content and high digestibility by fish (Baker and Gunther 2004). Diets containing 5 percent Spirulina platensis can replace Artemia nauplii in Litopenaeus schmitti larvae culture (Jaime-Ceballos et al. 2005). Algae contain 40- 60 percent protein, 7 percent mineral salts, chlorophyll, bile pigments, fiber and nucleic acid content (4-6 percent). Production of algae is TABLE 1. Nutrient composition (as percent dry weight) of the main group of microorganisms used for single- cell protein production (Miller and Litsky 1976). Component Fungi Algae Yeast Bacteria Protein 30-45 40-60 45-55 50-65 Fat 2-8 7-20 2-6 1-3 Ash 9-14 8-10 5-10 3-7 Nucleic acids 7-10 3-8 6-12 8-12 Substrate Fermentation Nutrient Filtration Drying SCP Submerged fermentation Semisolid fermentation FIGURE 1. Production of single-cell protein (SCP).